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View Full Version : Does "case trim" length affect accuracy??


Jicko
08-14-2007, 3:19 PM
I think this is a general question, but for me, this applies to .308

I am wondering, if variation in "case trim" will affect accuracy/pressure/muzzle velocity etc?

Generally I see a recommend "trim to" length of .308 being 2.005", will a batch of cases varying from 2.002" to 2.008" be ok to use together? Or is this one of those "very critical" attribute that one should only group exact length cases together??

And since i don't trim "every case" at the moment, can I mix in cases that are over 2.010" and still under the limit 2.015"?

rksimple
08-14-2007, 4:19 PM
I think this should be in the reloading section...but I'll take a stab at it.:D

Hard-core accuracy guys will always get them as close as possible to keep neck tension as consistent as possible. I usually trim after the first firing to even things out and square up the case mouth, but I don't trim for about 4 firings after that.

FWIW , here is a group using new, untrimmed, brass. My ES is about 14. I'll have to look back through my records, but it really surprised me.

http://i17.photobucket.com/albums/b82/rksimple/IMG_0003.jpg

Rule .308
08-14-2007, 4:21 PM
That is a good question, for which I really do not have an answer but I can tell you that one of the main keys to accuracy is consistancy. The idea is to remove all of the variables and have every round be as close to exactly like the next one as possible. Are you a good enough shooter and is your rifle good enough to really wring out the difference?

PistolPete75
08-14-2007, 4:24 PM
That is a good question, for which I really do not have an answer but I can tell you that one of the main keys to accuracy is consistancy. The idea is to remove all of the variables and have every round be as close to exactly like the next one as possible. Are you a good enough shooter and is your rifle good enough to really wring out the difference?

+1 consistancy. 2.005 is the min, 2.015 is the max. try 2.007, so you can get a little margain of error of cutting in case you trim too much.

also you will have to reset your size die. don't make the same mistake that i did!

i think your on the same press that i have. get the dillon 1200 trimmer~! trimming is soooo easy with it, once you get it set up right.

Jicko
08-14-2007, 4:42 PM
Oops.... can one of the mod move this to the "Reloading" section? I dunno what happened... I intended to post it there...

Jicko
08-14-2007, 4:49 PM
+1 consistancy..

I understand the strive for "consistancy"..... but I just wonder, if there are documented effect.... or known intelligence in terms of how "critical" this variable is....

Something like consistant powder drop wt., that is a very "critical" thing.... if one has large variation in the amount of powder they put in each case, that have a huge effect.

Something like consistant bullet length.... I dunno how "critical" that is(I don't think it is all that "critical" personally), but if you do measure a box of bullets, they do vary in lengths...



also you will have to reset your size die. don't make the same mistake that i did!

Please elaborate.


i think your on the same press that i have. get the dillon 1200 trimmer~! trimming is soooo easy with it, once you get it set up right.

I've both a Dillon RL550B as well as a RCBS Rock Chucker 2.... I am using the Rock Chucker 2 to do my .308 reloading. I am using a Lyman Accutrim, which is very... NOT "accurate".... *sigh*

Pthfndr
08-14-2007, 7:58 PM
RK has it correct. It's all about consistency. How much difference it makes depends just how accurate you need to be. For benchrest guys it makes a BIG difference. For Palma shooters, or anyone else shooting 1000 yards it can make a pretty good difference.

But.....

There's trimming to length and trimming to length. You could trim your cases to the same OAL, but not have the same length on the neck, which could cause a difference in neck tension from round to round.

I'll let an expert explain better. The following is from Doug Giraud, who makes the famed Giraud Trimmer which is arguably the best one out there.


When a trimmer grips the case rim, like the RCBS unit, it will control the distance from the rim to the case mouth to an exact length. The downside is that there is no guarantee that the brass has been resized, or resized properly to any specific given dimension. That type of trimmer, as well as the Lee, Forester, or any lathe type is that they don't care about what is between the rim and case mouth. Your brass could be undersized, oversized, some combination of both, and/or not resized at all.

With the Giraud and the Gracey, the case holders are indexing the cases off the shoulder of the case. These trimmers will trim the brass from the shoulder forward to a given dimension, but not necessarily the exact same dimension from case to case if the headspace dimensions vary. The headspace dimension is the distance from the case rim to the midpoint on the case shoulder. This will control how tightly or loosely the case will fit in the rifle chamber. If the headspace is looser than it should be, the case will be sloppy in the chamber, if it is too long, you may have troubles closing the bolt on a loaded round.

Theoretically, if the headspace was way short and the case was sloppy loose, even though the distance from the rim to the case mouth was right on call for SAAMI dimensions, the case could slide forward enough to allow the case mouth to get restricted by the end of the chamber and cause the typical problems associated with cases that are too long.

With the Giraud and Gracey style trimmers, the distance from the shoulder to the mouth is exactly the same from case to case and there is no way the mouth could be restricted by the end of the chamber if the case was trimmed properly. The shoulder of the case would prevent the case from going forward any more and causing problems. Where some people have a problem with these types of trimmers is that the overall length can vary by however much your headspace dimension varies. But that is a function of your ability to consistently resize cases, not the trimmer. The problem is that most everybody measures only the distance from the rim to the case mouth and sees some variation. They don't usually take the time to get a case gauge, like a Stoney Point comparator, and measure the distance from the rim to the shoulder and see that the variation is there, not from the shoulder forward to the case mouth.

Reloaders who have been making match grade ammo for competition or such usually have learned this the hard way and take more care when resizing brass to make the headspace more uniform. With brass that is consistently resized, the overall length from a Giraud or Gracey trimmer will usually be as uniform as your ability to control that headspace dimension.

Now all the trimmers have the ability to trim a .223 case to 1.750" on a regular basis. But if you don't know that the headspace on the case is .020" short and you only measure the rim to case mouth dimension, you don't know that your case is sliding .020" further into the chamber than you thought and may create problems. With the Giraud and Gracey trimmers, the case holders will typically not work well unless the brass has been resized, so there is one check you can make easily. Does the case go into and out of the case holder easily? If not, check it out. The you can check the headspace of a few cases and see if they are consistent. If they are, you can set the trimmer to remove enough material to know that the case will function properly, and the headspace will prevent the case from sliding around inside the chamber. And lastly, with proper headspace, you stand a better chance of getting accurate loads and longer life from your brass than if the headspace was excessive or random.

Sorry for being so long winded, but I am an engineer. And we all know what that means, I have low earning potential and poor social graces.

HTH,
Doug Giraud

rksimple
08-14-2007, 8:37 PM
Good info pthfndr. Doug's the man!

I got the Gracey trimmer as a gift. Otherwise, I would have got the Giraud as its much easier to adjust. Doyle Gracey, the guy who invented the trimmer, lives right around the corner from me. He's a nice old guy with all kinds of shooting stories and a lot of practical shooting experience. Lotsa trophies on the wall at his place. His trimmer was the first of its type on the market, and he's always glad to show you how consistent his trimmer is by doing a few cases for ya' right there in his shop.

Bottom line is make everything as consistent as practical. Shoot the stuff you have trimmed and see how it goes. It may be plenty good for the type of shooting you need it for.